How does an interface differ from an abstract class in Java?
Abstract class: If a class contains abstract methods, it is called abstract. Abstract classes must be preceded by the abstract keyword. You cannot create objects with abstract classes because they contain methods that have no concrete implementation. An abstract method is a special kind of method: it has declarations and no concrete implementation. You cannot create objects with abstract classes because they contain methods that have no concrete implementation. If a class inherits an abstract class, it must provide definitions for abstract methods in the base class. If you don’t, the exported class is also abstract.
Note that there are three main differences between abstract classes and ordinary classes:
1. The abstract method must be either public or protected (because if it is private, it cannot be inherited by subclasses and they cannot implement the method). By default, it is public. 2. Abstract classes cannot be used to create objects. 3. If a class inherits from an abstract class, the subclass must implement the abstract methods of the parent class. If a subclass does not implement the abstract methods of its parent, it must also be defined as an abstract class.
Interface: can contain variables and methods. Note, however, that variables in the interface are implicitly specified as public static final variables (and only public static final variables). Methods are implicitly specified as public abstract methods and only public abstract methods (using other keywords such as private, protected, static, final, etc.), and all methods in the interface cannot be implemented. That is, the methods in the interface must all be abstract methods. An interface is an extremely abstract type, more “abstract” than an abstract class, and generally does not define variables in the interface.
Abstract classes differ from interfaces
Abstract class represents an inheritance relationship in the Java language. A class can only use an inheritance relationship once. However, a single class can implement multiple interfaces. 2. An abstract class can have its own data members or non-abstract member methods, whereas an interface can only have static data members that cannot be modified. In interface, however, data members are not defined. All member methods are abstract. 3. A class that implements an abstract class or interface must implement all of its methods. If the subclass does not implement all of the abstract methods of the abstract class, the subclass must also be abstract. But if a subclass implements an interface, it must implement all of the interface’s abstract methods. Interfaces can also inherit from interfaces, so there is no need to implement a superclass interface. Abstract classes can have non-abstract methods. There can be no implementation methods in the interface. 4. Variables defined in the interface default to public static final, and must be given an initial value, so the implementation class cannot redefine or change its value. 5. Methods in interfaces are public abstract by default. Static methods are class methods that do not allow subclass overrides. But abstract classes allow static methods.